As a thermal transfer sheet in a formation method of images using thermal transfer, a sublimation dye thermal transfer sheet in which a sublimation dye transfer ink layer composed of a sublimation dye and a binder is provided on one surface of a base film as a color material layer, and a thermofusible thermal transfer sheet in which a thermofusible transfer ink layer composed of a pigment and a wax is provided in place of the sublimation dye transfer ink layer are known. In these thermal transfer sheets, a protective layer to be transferred to a thermal transfer image-receiving sheet can also be further provided on the same face as in the color material layer of the base film as required.
In the thermal transfer sheet, generally, a heat resistant slipping layer (also referred to as a back layer) is provided on a surface opposite to the surface of a base film on which a color material layer is provided in order to stand heat energy from a thermal head, but when the thermal transfer sheet is stored in a wound state after printing, the color material layer is brought into contact with the heat resistant slipping layer and pressed against the heat resistant slipping layer under a pressure, and therefore a dye in the color material layer may be transferred (kick) to the heat resistant slipping layer. If the thermal transfer sheet is cut and rewound for processing the thermal transfer sheet into a finished product in such a state in which the dye of the color material layer has been transferred to the heat resistant slipping layer, the dye transferred to the heat resistant slipping layer may be transferred to other color material layer adjacent to this dye, that is, retransfer may occur (back). If the retransfer occurs like this, an image-receiving sheet has different color hue from designated color when the color material layer contaminated with the retransfer is thermally transferred to the image-receiving sheet and printing precision is significantly impaired. This is further remarkable in a case where a transfer protective layer is provided beside the color material layer. That is, since this protective layer is a transparent film to avoid impairing image characteristics, the dye is retransferred to this transparent film, and when this contaminated transparent film is transferred on the images as a protective film, the contamination due to the dye is further emphasized to impair the printing precision significantly.
On the other hand, it is required that the heat energy of the thermal head is increased, the sensitivity of an ink ribbon is enhanced, the content of a dye is increased and the like with accompanying speeding up of printers in recent years, but these changes increase the possibility of transferring the dye from the color material layer to the heat resistant slipping layer and causing troubles during thermal transfer resulting from this transfer. Therefore, requirements for the heat resistant slipping layer to reduce dye retransfer have been further increased.
As a heat resistant slipping layer reducing the dye retransfer, there are proposed, for example, a heat resistant slipping layer (for example, see patent document 1) which contains a phosphate ester having a melting point of 35° C. or higher in an amount of 5 to 50 parts by weight and contains a polyvinyl acetal resin having a glass transition temperature of 80° C. or higher; a heat resistant slipping layer (for example, see patent document 2) which contains a binder resin such as a thermoplastic resin, a lubricant having a thermal cracking temperature of 200° C. or higher, and particles having a Mohs' hardness of less than 3 and reduces wear of the thermal head; a heat resistant slipping layer (for example, see patent document 3) which is predominantly formed of a reaction product of an active hydrogen-containing thermoplastic resin such as a polyvinyl butyral resin with isocyanate and is superior in an antistatic property; and a heat resistant slipping layer (for example, see patent document 4) which contains a natural organic polymer powder and molybdenum disulfide and can prevent wear of the surface of the thermal head. However, a thermal transfer sheet in which the dye retransfer is reduced by selecting a cellulosic resin as a binder resin of the heat resistant slipping layer is not described in any patent document.
Furthermore, there are also proposed a heat resistant slipping layer (for example, see patent document 5) which includes a cellulose acetate butyrate resin containing 5 to 50% of a propyl group and 10 to 45% of a butyl group and improves heat resistance; a heat resistant slipping layer (for example, see patent document 6) in which roughness (SRz) is limited to 3.0 μm or more; and a heat resistant slipping layer (for example, see patent document 7) formed of a mixture of a heat resistant resin, a lubricant having a melting point of 33° C. or higher and an IO value of 0.23 or more, and a polyisocyanate compound having two or more isocyanate groups in its molecule; but it is not described and suggested that the content of a butyryl group in the cellulose acetate butyrate resin is specified with respect to reduction of the dye retransfer.    Patent Document 1: Japanese Kokai Publication Hei-9-300827    Patent Document 2: Japanese Kokai Publication Hei-6-247065    Patent Document 3: Japanese Kokai Publication Hei-7-149062    Patent Document 4: Japanese Kokai Publication 2000-255172    Patent Document 5: Japanese Kokai Publication Hei-1-234292    Patent Document 6: Japanese Kokai Publication 2000-225775    Patent Document 7: Japanese Kokai Publication 2002-11967